OpenEuphoria

Re: Last Element Reference

• Posted by Robert Craig <rds at RapidEuphoria.com> Sep 19, 2003
• 478 views

Derek Parnell wrote:
> If I may, I'd like to supply a requirements document for the proposed 
> enhancement to Euphoria.
> 
> Title: Last Element Reference
> 
> Functional Requirement:
> To enable a coder to reference the last element in a sequence, within 
> slice specifications and element references, without having to 
> explicitly name the sequence itself.
> 
> Benefits:
> * Reduce the amount of typing.
> * Reduce the possibility of misspelling a identifier.
> * Improve legibility by reducing the number of characters to read.
> * Improve interpreter performance by having less tokens to process.
> 
> Costs:
> * If using a new syntax symbol, it will reduce the number of available 
> symbols for future use.
> 
> Technical Specification:
> I suggest that the character '$' be used as a shorthand for 
> "length(<seq>)" when '$' appears inside a slice specification or inside 
> an element reference. And that the sequence it refers to is the same one 
> that the slice and element reference is using.
> 
> Examples:
> 
>   sequence X
> 
>   X = {"ant", "bear", "cat", "dog", "elephant"}
> 
>   ? X[$] --> "elephant"
>   ? X[$][$] --> 't'
>   ? X[3 .. $] --> {"cat", "dog", "elephant"}
>   ? X[3 .. $-1] --> {"cat", "dog"}
>   ? X[$-1..$] --> {"dog", "elephant"}
>   ? X[$][3..$] --> "ephant"
>   ? X[$][$-3..$] --> "hant"
>   ? X[2][$] --> 'r'
>   ? X[1 .. $-1] --> {"ant", "bear", "cat", "dog"}
>   ? X[1 .. floor($/2)] --> {"ant", "bear"}
>   ? X[floor($/2)+1 .. $] --> {"cat", "dog", "elephant"}

Yes. That's exactly what I was thinking.
There are some weird cases too, like:

    x[y[$]..$]        -- $ refers to the immediately enclosing brackets

    x[$ * $ - foo($)] -- $ could be used anywhere, in any way
                      -- inside the brackets

$ would be a kind of very localized constant
with a scope from [ to ]. It's value would be
the length of the sequence being subscripted or sliced.

Pete Lomax wrote:
 > Since you asked, I'm [still] sold on negative indexes:
 >
 >	s="(0,0)"
 >	x=s[1..-1]		-- x is now "(0,0)"
 >	x=s[2..-2]		-- x is now  "0,0"
 >	x=s[3..-3]		-- x is now   ","
 >
 >	s="abc.exw"
 >	if equal(s[1..3],"abc") then		-- this is true
 >	if equal(s[-3..-1],"exw") then		-- this is true
 >
 >	s=s[3..-1]		-- removes the first two elements of s
 >	s=s[1..-3]		-- removes the last two elements of s
 >
 >	s=s[1..2]		-- selects the first two elements of s
 >	s=s[-2..-1] 	-- selects the last two elements of s

I find negative subscripts to be a bit less clear.
Plus I'd like to continue to be able to catch all
negative numbers as bad subscripts.

 > "end" may be a more readable alternative to $.
 > When "end" is between [] should be no major parsing problem?

"end" could be parsed easily enough using the RDS parser,
but I think it would be visually confusing for a person
reading the code, especially a beginner. He might
confuse it with "end if", "end for", "end procedure" etc.,
or he might think "end" was a variable, and might wonder
where it was assigned or declared.

Regards,
    Rob Craig
    Rapid Deployment Software
    http://www.RapidEuphoria.com